1. Field of the Invention
The present invention relates to a top plate, positioning apparatus, exposure apparatus, and device manufacturing method and, more particularly, to a top plate having a hollow structure, a positioning apparatus including the top plate, an exposure apparatus including the positioning apparatus, and a device manufacturing method utilizing the exposure apparatus.
2. Description of the Related Art
FIG. 10 is a schematic perspective view of a positioning apparatus formed as a wafer stage apparatus. A slider surface plate 75 is mounted at the center of a base surface plate 79, and X surface plates 78X and Y surface plates 78Y are mounted around the slider surface plate 75. Stators 77X of coarse linear motors which drive an X beam 73X in the X direction are mounted on the X surface plates 78X, respectively. Stators 77Y of coarse linear motors which drive a Y beam 73Y in the Y direction are mounted on the Y surface plates 78Y, respectively.
The X beam 73X and Y beam 73Y are arranged to intersect each other and extend through an X-Y slider 72. The X beam 73X and Y beam 73Y drive the X-Y slider 72 in the X and Y directions while maintaining a non-contact state with the X-Y slider 72. A six-axis fine stage 70 is mounted on the X-Y slider 72. A wafer chuck is mounted on the six-axis fine stage 70.
The Y beam 73Y has Y feet 74Y, to which static pressure air bearings (not shown) attach, at its two ends. The slider surface plate 75 supports the Y beam 73Y in the vertical direction (Z-axis direction) through the static pressure air bearings. The X beam 73X has an X foot 74X and X foot 74X′ at its two ends. The slider surface plate 75 supports the X beam 73X in the vertical direction (Z-axis direction) through static pressure air bearings. A Y guide 76 attached to the slider surface plate 75 guides the X foot 74X′ in the horizontal direction (Y-axis direction) through static pressure air bearings (not shown). The slider surface plate 75 supports the X-Y slider 72 in the vertical direction (Z-axis direction) through a static pressure air bearing (not shown) attached to its bottom surface.
FIG. 11A is a plan view seen from above the fine stage 70, FIG. 11B is a plan view seen from above a fine stage fixed plate 702, and FIG. 11C is a plan view seen from below a top plate 701. The fine stage 70 comprises the fine stage fixed plate 702 and top plate 701. A self-weight support spring (not shown) extends between the fine stage fixed plate 702 and top plate 701 to support the weight of the top plate 701. Fine linear motors are arranged between the fine stage fixed plate 702 and top plate 701. Stators 703Xa, 703Ya, and 703Za of the fine linear motors which have coils fixed to the fine stage fixed plate 702. Movable elements 703Xb, 703Yb, and 703Zb of the fine linear motors which have magnets are fixed to the top plate 701. The stators 703Xa and movable elements 703Xb generate thrusts in the X direction. The stators 703Ya and movable elements 703Yb generate thrusts in the Y direction. The stators 703Za and movable elements 703Zb generate thrusts in the Z direction.
Magnetic plates 705 are arranged on the side surfaces of the four sides of the top plate 701 through attaching plates 704. E-shaped electromagnets 707 having coils are arranged on the fine stage fixed plate 702 through attaching plates. When supplying a current to the coils, attracting forces are generated between the E-shaped electromagnets 707 and magnetic plates 705 to transmit the acceleration/deceleration force of the X-Y slider 72 to the top plate 701. The E-shaped electromagnets 707 and magnetic plates 705 thus serve as electromagnetic couplings. The opposing surfaces of the E-shaped electromagnets 707 and magnetic plates 705 form arcs having their centers at the rotation center of the top plate 701. By employing this arc shape, the E-shaped electromagnets 707 and magnetic plates 705 can rotate freely about the Z-axis without coming into contact with each other. During rotation, the gaps between the E-shaped electromagnets 707 and magnetic plates 705 do not change, and the attracting forces generated by the electromagnets with respect to same current do not change. The lines of action generated between the E-shaped electromagnets 707 and magnetic plates 705 preferably run through the barycentric position of the whole movable body of the fine stage 70 including the top plate 701, the linear motors attaching to it, and the like.
The top plate 701 has a hollow rib structure to decrease the weight and increase the rigidity. FIG. 12 is a sectional view showing a typical rib structure. In FIG. 12, ribs R701 form a rhombus. As disclosed in Japanese Patent Laid-Open No. 2003-163257, this can increase the natural value of the torsion mode as the primary mode of the top plate 701 to obtain high-speed, high-accuracy followability of the stage.
In recent years, higher-speed positioning has been required, and accordingly further weight reduction has been required of the top plate as part of the movable portion of a fine stage. For this purpose, a thickness t701 of each rib R701 as shown in FIG. 12 must be decreased as much as possible. If, however, the rib is excessively thin, even if the natural value of the global mode such as the torsion of the top plate may be maintained, the natural value of the local mode of the rib itself as indicated by broken lines in FIG. 12 decreases to adversely affect the followability of the stage.
Japanese Patent Laid-Open No. 2004-254489 discloses a plane stage in which driving forces in the direction of six degrees of freedom are generated between the coils of the stator and the magnets of the movable element to realize a large stroke, high-accuracy positioning, and high-accuracy posture control. A top plate for such a plane stage preferably comprises a three-pin support mechanism to transfer a wafer from a transfer hand to a chuck held by the top plate. This increases the thickness of the top plate. Accordingly, the natural value of the local mode of the rib further decreases, and boring to form the rib becomes difficult.